Code-division-multiple-access mobile communication system accommodating increased number of mobile stations

ABSTRACT

A system for mobile communication based on code division multiple access includes base stations, each of which communicates with mobile stations by using a plurality of radio frequencies covering respective cells, the respective cells including a first cell covered by a first radio frequency and a second cell covered by a second radio frequency. The system further includes a base-station controller which communicates with the base stations, and controls the mobile stations to switch from the first cell of a first base station to the first cell of a second base station via a soft hand-off operation and switch between the first cell and the second cell within any base station via a hard hand-off operation, the base-station controller providing the mobile stations with no direct switch between the second cell of the first base station and the second cell of the second base station.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cell designs of a mobile communicationsystem that is based on a CDMA (code division multiple access) schemetypically used in an IS-95-A scheme.

2. Description of the Related Art

As a number of customers increases in a mobile-communication system,there is an increasing need for a system that can accommodate a largenumber of customers.

FIG. 10 is an illustrative drawing showing-a configuration of a typicalrelated-art mobile-communication system.

In the system of FIG. 10, a public telephone network is connected to amobile network via mobile switch center MSC. The mobile switch centerMSC has base station controllers BSC connected thereto, and the basestation controllers BSC in turn have base stations BTS connectedthereto. Each of the base stations BTS communicates with mobile stationsMS residing in its cell (i.e., area of control) so as to render servicessuch as a telephone service. In such a mobile-communication system, aCDMA (code division multiple access) scheme, a TDMA (time divisionmultiple access) scheme, or a FDMA (frequency division multiple access)scheme is typically employed for the purpose of providing multipleaccesses.

[CDMA Scheme]

The CDMA scheme is used in the IS-95-A scheme. In the CDMA scheme, abase station uses the same frequency for communicating with differentmobile stations residing in its own cell. Channels for communicatingwith respective mobile stations are established by using predeterminedcodes, which are called dispersion codes, and serve to discriminaterespective signals of mobile stations. Data exchanged between the basestation and a mobile station is encrypted (frequency dispersed) byconvolving the data with a dispersion code. On the receiver side, thereceived data is further convolved with the same dispersion code inorder to identify a channel.

In the CDMA scheme, a transmitter side of a base station uses two typesof dispersion codes. One is a short code, which is used fordiscriminating the base station from other base stations. The other is along code, which is used for discriminating a mobile station as adestination. These two codes are convolved with transmission data.

Further, a transmitter side of a mobile station uses two types ofdispersion codes. One is a short code again, which is used by a basestation for obtaining a data timing of data received from the mobilestation. The other is a long code, which serves to discriminate themobile station from other mobile stations. These two codes are convolvedwith transmission data.

Such dispersion codes as described above are used forchannel-discrimination purposes in the. CDMA scheme. Because of this,each mobile station can selectively pick up a channel directed to itselffrom a relevant base station even when each mobile stationsimultaneously receives signals of the same radio frequency from aplurality of base stations.

In this manner, the CDMA scheme allows base stations to transmit thesame frequency to all the mobile stations, and allows all the mobilestations to transmit the same frequency to the base stations. Pleasenote, however, that the transmission frequency of the base stations isdifferent from the transmission frequency of the mobile stations.

[Hand-off of CDMA Scheme]

“Hand-off” refers to an operation performed when a mobile station movesfrom a cell of a given base station to a cell of an adjacent basestation while engaging in a call. The CDMA scheme performs a softhand-off operation to insure a continuous call without a break.

During a period of a soft hand-off state, two base stations havingbordering cells transmit the same data received from the base-stationcontroller to a mobile station currently positioned around the border ofthe cells. The mobile station combines the received signals sent fromthe two base stations, thereby improving a reception gain. Each of thetwo base stations receives a signal sent from the mobile station, andforwards the signal to the base-station controller. The base-stationcontroller compares the two signals sent from the two respective basestations, and select one having a better signal quality. Selected datais then sent to the mobile-switch center. In this manner, a call neverbreaks during a soft hand-off period as long as either one of the twobase stations securely receives signals from the mobile station.

A mobile-communication system based on the TDMA scheme typically employsa different type of a hand-off operation called a hard hand-off. In ahard hand-off operation, a radio frequency is switched after a mobilestation comes sufficiently close to a first base station when movingfrom a second base station to the first base station with an aim ofachieving a secure shift. This means, however, that the mobile stationbecomes distanced from the second base station before the hand-offoperation is actually performed. A hard hand-off thus requires a greatertransmission power than a soft hand-off. Further, a communicationsuffers a brief moment of disconnection at the time of switching.

Even the CDMA scheme may use a hard hand-off operation when two basestations cannot use the same frequency to provide respective services toa mobile station, for example. In such a case, a brief moment ofdisconnection is observed before a switched channel is reconnected.

[Number of Subscribers in CDMA]

The CDMA scheme achieves division of channels by use of codes, and usesthe same radio frequency shared by a large number of mobile stations.When a base station attempts to receive a signal from a given mobilestation, other signals transmitted from other mobile stations using thesame radio frequency appear to be nothing but sources of interferencesfor the base station. Namely, an increase in the number of mobilestations adding to the number of transmission signals is tantamount toan increase in noise. The acceptable number of mobile stations that cancommunicate using the same radio frequency is obviously limited by thedegree of interference. It is important, therefore, to reduceinterferences by using as small transmission power as possible for eachmobile station. This is the most important issue to be addressed indeciding the number of mobile stations than can be accommodated in thesame cell, i.e., the number of customers of a single system.

In order for a mobile station to reduce its transmission power around aborder of cells, a soft hand-off is suitable because it requires only aminimum transmission power that achieves communication with the closestbase station.

As a mobile station shifts its position, a building may come into a linebetween the mobile station and the base station, or may go out of theline. When the mobile station is obscured by a building, the basestation in the CDMA system increases transmission power in response toweakening signals if the CDMA system is not using a soft hand-off. Suchan increase in transmission power is an increase of noises as far asother mobile stations are concerned. When the mobile station comes outfrom behind the building, the transmission power is decreased. Such anadjustment of transmission power is repeated as the mobile stationmoves.

In a system which employs a soft hand-off, even when a base station isobscured by a building, a mobile station may maintain a connection withanother mobile station. In such a case, necessary transmission power issmaller compared to the case of no soft hand-off operation. Namely, anoise effect on other mobile stations is smaller.

Accordingly, a system employing the soft hand-off operation canaccommodate a larger number of mobile stations than a system using nosoft hand-off, thereby achieving a smaller system cost per user.

[System Configuration of CDMA Scheme]

FIG. 11 is an illustrative drawing showing a configuration of areas(cells) of related-art base stations employing the CDMA scheme.

As previously described, the number of channels that a single basestation can use with a common radio frequency is limited by an effect ofsignal interference. When the number of customers (mobile stations) islarger than the number of channels that can be accommodated by the samefrequency, a cell configuration is designed such that a single basestation uses different radio frequencies for implementing a plurality ofcells. For example, a base station that renders services to more mobilestations than an acceptable number of mobile stations for a single radiofrequency needs to implement cells using different radio frequencies.

As shown in FIG. 11, a base station 1 implements a plurality of cells byusing a plurality of radio frequencies RF1, RF2, and RF3. Areas coveredby the respective radio frequencies RF1, RF2, and RF3 are completelyoverlapped, and encompass the base station 1 with a radius R1. Further,the areas of the respective radio frequencies RF1, RF2, and RF3 of thebase station 1 partially overlap corresponding areas of respective radiofrequencies RF1, RF2, and RF3 of a base station 2. This partialoverlapping is provided in order to permit a soft hand-off operationbetween areas using the same radio frequency.

[Selection of Soft Hand-Off or Hard Hand-Off]

In the CDMA scheme, a decision has be to made as to which one of thesoft hand-off and the hard hand-off is used at a border of adjoiningcells. To this end, a mobile station obtains the following thresholdvalues from a base station.

1) pilot strength usable for communication

2) pilot strength to trigger hand-off

3) pilot strength lower than the above

A mobile station starts communicating with a base station for locationupdate or the like when finding this base station before any other basestations by picking up a signal from this base station that exceeds“pilot strength usable for communication”.

If a user of the mobile station requests a call, the mobile stationsends a call request to the base station. A mobile station constantlysearches for pilot channels of surrounding cells, and monitors receivedstrengths of the pilot channels. If any one of the received strengthscrosses over from one category to another category classified by theabove conditions 1) through 3), the mobile station reports the receivedstrengths of pilot channels to the base-station controller via thecurrently connected base station.

Based on the reported strengths of pilot channels of surrounding cells,the base-station controller selects one of the following operations.

1) soft hand-off

2) hard hand-off

3) maintain current state

If a soft hand-off or a hard hand-off is selected, a hand-off switchmessage is sent to the mobile station, thereby prompting the mobilestation to switch over to one of the surrounding cells.

In this process, a decision as to which one of the two hand-offoperations is selected is made by taking into account the followingfactors.

1) soft Hand-Off

Conditions that must be satisfied in order to select a soft hand-off areas follows:

a received pilot strength of a surrounding cell that is reported by themobile station exceeds “pilot strength usable for communication”; and

a target cell (a surrounding cell that is currently evaluated) has anavailable resource for the same frequency and the same frame offset asthose of the currently used cell.

Such a soft hand-off achieves a switch to the target cell using the sameradio frequency and the same frame offset as those of the currently usedcell.

In the example of FIG. 11, each of the base stations 1 and 2 uses theradio frequencies RF1, RF2, and RF3 to communicate with mobile stations.Even though a plurality of the radio frequencies RF1, RF2, and RF3 areused, overlapping is provided between the cells using the samefrequency. A soft hand-off thus can be performed for a mobile station 3between the cells using the same frequency.

The frame offset refers to a position in a series of time slots that areused for exchanging communication signals of mobile stations between abase station and a base-station controller on a communication lineutilizing a time-division multiplex scheme. A soft hand-off can not beperformed unless a position of a time slot of a mobile station is thesame in a base station after a hand-off as was in a base station beforethe hand-off. Therefore, a check has to be made as to whether a frameoffset (i.e., a particular time slot) used in a base station before ahand-off is available in a base station to be used after the hand-off.That is, whether the same frame offset is available in the base stationto be used needs to be checked in order to perform a soft hand-offoperation.

2) Hard Hand-Off

Conditions that must be satisfied in order to select a hard hand-off areas follows:

a received pilot strength of a surrounding cell that is reported by themobile station exceeds “pilot strength to trigger hand-off”;

a pilot strength of a currently used cell is below “pilot strengthusable for communication”;

a target cell has available resources; and

the target cell does not have an available space for the same frequencyand the same frame offset as those of the currently used cell.

A hard hand-off may include a case where a switch is made to a differentradio frequency when moving into a target cell or a case where a switchis made to a different frame offset while using the same radiofrequency.

3) Maintaining a Current Status

Conditions that must be satisfied in order to maintain a current statusare as follows.

a received pilot strength of a surrounding cell that is reported by themobile station exceeds “pilot strength to trigger hand-off”.

a pilot strength of a currently used cell is above “pilot strengthusable for communication”; and

a target cell has no available resources, or does not have an available,space for the same frequency and the same frame offset as those of thecurrently used cell.

When a decision is made to keep a current status, no hand-off isperformed, and a connection with the current base station remains as itis.

In this manner, a hand-off operation is performed by evaluating receivedpilot strengths that are reported to a base-station controller from amobile station. Decisions as to whether to perform a hand-off operationand which type of hand-off operation is to be performed are made by thebase-station controller. To this end, the base-station controller needsto keep track of locations of and frequencies used by all the mobilestations.

[Details of Hard Hand-Off in CDMA]

FIG. 12 is an illustrative drawing showing a hard hand-off operationperformed by a mobile station.

In FIG. 12, the base stations 1 and 2 are under the control of abase-station controller 4. Ellipses drawn above the base stations 1 and2 illustrate cells (areas) covered by the radio frequencies RF1 and RF2.Points a through f indicate positions of the mobile station 3. What isshown in the middle of the figure demonstrates pilot strengths of thebase stations 1 and 2 that are received by the mobile station 3 as itmoves along. In this presentation, a pilot strength x indicates a “pilotstrength usable for communication”, and a pilot strength y indicates a“pilot strength to trigger a hand off”.

In the following, a series of operations from when the mobile station 3starts communication with the base station 1 at the point a by using theradio frequency RF1 to when the mobile station 3 finally reaches thepoint f will be described.

When the mobile station 3 reaches the point c, the received pilotstrength of the base station 2 exceeds the pilot strength y (i.e.,“pilot strength to trigger a hand-off”). The mobile station 3 reportsthis change to the base-station controller 4 via the base station 1.

The base-station controller 4 makes a resource request to the basestation 2 with an aim of performing a soft hand-off operation. In thisexample, however, there is no resources, and a current status ismaintained.

When the mobile station 3 moves to the point e, the received pilotstrength of the base station 1 becomes smaller than the pilot strength x(i.e., “pilot strength usable for communication). The mobile station 3reports this to the base-station controller 4 via the base station 1.The base-station controller 4 instructs the base station 1, the basestation 2, and the mobile station 3 to carry out a hard hand-offoperation. The hard hand-off operation is carried out at the point e. Inthis manner, the mobile station 3 communicates with the base station 1from the point a to the point e, and communicates with the base station2 from the point e to the point f.

[Details of Soft Hand-Off in CDMA]

FIG. 13 is an illustrative drawing showing a soft hand-off operationperformed by a mobile station. In FIG. 13, the same numerals and symbolsas those of FIG. 12 are used for referring to the same items.

In the following, a series of operations from when the mobile station 3starts communication with the base station 1 at the point a by using theradio frequency RF1 to when the mobile station 3 finally reaches thepoint f will be described.

When the mobile station 3 reaches the point c, the received pilotstrength of the base station 2 exceeds the pilot strength y (i.e.,“pilot strength to trigger a hand-off”). The mobile station 3 reportsthis change to the base-station controller 4 via the base station 1. Thebase-station controller 4 makes a resource request to the base station 2with an aim of performing a soft hand-off operation. When resources aresecured, the base-station controller 4 instructs the base stations 1 and2 and the mobile station 3 to carry out a soft hand-off operation, sothat the mobile station 3 starts communicating with both of the basestations 1 and 2.

When the mobile station 3 moves to the point e, the received pilotstrength of the base station 1 becomes smaller than the pilot strength x(i.e., “pilot strength usable for communication). The mobile station 3reports this to the base-station controller 4 via the base stations 1and 2. The base-station controller 4 instructs the base station 1, thebase station 2, and the mobile station 3 to end the soft hand-offoperation. As a result, the mobile station 3 communicates only with thebase station 2. In this manner, the mobile station 3 communicates withthe base station 1 from the point a to the point e, and communicateswith the base station 2 from the point c to the point f. Between thepoint c and the point e, the soft hand-off operation is being engaged,allowing simultaneous communications with the two base stations.

[Configuration of Base Station and Base-Station Controller]

FIG. 14 is a block diagram showing a related-art configuration of a basestation and a base-station controller.

The base station includes a plurality of identical configurations asmany as there are used radio frequencies (i.e., three in this examplesince three radio frequencies RF1, RF2, and RF3 are used).

The base station is provided with two antennas with respect to eachradio frequency for signal exchanges with mobile stations. One antennais used for transmission of signals, and the other antenna is used forreceiving signals.

On a receiver side, RF-conversion units 30 through 303 convert a radiosignal received by the antenna into an intermediate frequency signal,which is then demodulated by a QPSK-modulation/demodulation unit 31before being sent to CDMA-modulation/demodulation units 32 ₀ through 32_(n). The CDMA-modulation/demodulation units 32 ₀ through 32 _(n) areprovided as many as there are mobile stations that can communicatesimultaneously with the base station. In this example, therefore, thebase station can establish simultaneous communications with n+1 mobilestations. The CDMA-modulation/demodulation units 32 ₀ through 32 _(n)convolve the received signals with dispersion codes so as to attend toan inverse-dispersion process of the CDMA signals. The dispersion codesare determined by a BTS-control unit 33 in advance. A BSC-connectionunit 34 receives the received signals having the inverse-dispersionprocess applied thereto, and forwards the them to the base-stationcontroller.

On a transmitter side, the BSC-connection unit 34 receives transmissiondata from the base-station controller, and sends it to one of theCDMA-modulation/demodulation units 32 ₀ through 32 _(n) selected inadvance by the BTS-control unit 33. The selected one of theCDMA-modulation/demodulation units 32 ₀ through 32 _(n) convolves thetransmission data with a dispersion code to attend to a CDMA-dispersionprocess. Further, the QPSK-modulation/demodulation unit 31 applies aQPSK modulation to generate an intermediate frequency signal. One of theRF-conversion units 301 through 303 converts the intermediate signalinto a radio transmission signal, and transmits it via the antenna.

FIG. 15 is a block diagram of a RF-conversion unit 30 of the basestation. The RF-conversion unit 30 is any one of the RF-conversion units301 through 303.

On the receiver side of the RF-conversion unit 30, a band-pass filter301 filters a received radio signal, and, then, a low-noise amplifier302 amplifies the filtered signal. A multiplier 303 multiplies theamplified signal by an output of a receiver local-signal generator 306-1to obtain an intermediate frequency signal.

On a transmitter side of the RF-conversion unit 30, an intermediatefrequency signal is filtered by a band-pass filter 304. A multiplier 305multiplies the filtered signal by an output of a transmitterlocal-signal generator 306-2 to generate a radio transmission signal.The radios transmission signal is amplified by a high-power amplifier308, and, then, is transmitted from the antenna.

With reference to FIG. 14 again, on a receiver side of the base-stationcontroller, data sent from a plurality of base stations are received bya BTS-connection unit 11, and are provided to a communication settingunit 12. The communication setting unit 12 supplies the received data tocorresponding selection units 13 ₀ through 13 _(m) as a given chunk ofthe received data has an allocated selection unit. This allocation isdetermined by a BSC-control unit 16. Each of the selection units 13 ₀through 13 _(m) selects one of the two received data chunks that hasfewer errors than the other during a period of a soft hand-offoperation, and, then, applies an audio-decoding process before sendingthe selected data to a MSC-connection unit 15. The MSC-connection unit15 combines data supplied from the selection units 13 ₀ through 13 _(m)to generate frames, and sends these frames to a mobile-switch center 5.

On a transmitter side of the base-station controller, frames receivedfrom the mobile-switch center 5 are processed to extract transmissiondata, which is then sent to one of the selection units 13 ₀ through 13_(m) that is preselected by the BSC-control unit 16. The one of theselection units 13 ₀ through 13 _(m) applies an audio-coding processbefore sending the transmission data to the communication setting unit12. The transmission data is then transmitted via the BTS-connectionunit 11 to a destination that is specified by the BSC-control unit 16.

[Configuration of Selection Unit]

FIG. 16 is a block diagram of a selection unit of the base-stationcontroller. The selection unit 13 of FIG. 16 is any one of the selectionunits 13 ₀ through 13 _(m).

The selection unit 13 includes a first buffer 131, a second buffer 132,a third buffer 133, an audio decoding unit 134, an audio coding unit135, a buffer-control unit 136, a demultiplexer 137, a first check unit138, a second check unit 139, and a selector 140.

On a receiver side of the selection unit 13, the demultiplexer 137receives data, and supplies a first one of two data chunks consecutivelyreceived to the first check unit 138 and a second one of the two datachunks to the second check unit 139 if a soft hand-off operation isbeing engaged. The buffer-control unit 136 is notified when the datatransfer is completed. The first check unit 138 and the second checkunit 139 check errors in the received data, and send the received datato the first buffer 131 and the second buffer 132, respectively. Resultsof the error checks are provided to the buffer-control unit 136. Thebuffer-control unit 136 controls the selector 140 to select one of thetwo data chunks that has the smallest errors, and controls acorresponding one of the first buffer 131 and the second buffer 132 tosupply the received data to the audio decoding unit 134. Theseoperations as described above are repeated for each frame. If the softhand-off operation is not being engaged, the demultiplexer 137 suppliesdata to the first check unit 138 as it receives the data.

On a transmitter side of the selection unit 13, the audio coding unit135 applies audio-coding processing to transmission data, and sends theprocessed transmission data to the third buffer 133. Under the controlof the buffer-control unit 136, the third buffer 133 supplies thetransmission data to the communication setting unit 12

[Configuration of Mobile Station]

FIG. 17 is a block diagram of a receiver portion of a related-art mobilestation.

The mobile station of FIG. 17 includes a RF-conversion unit 21, aQPSK-demodulation unit 22, a searcher 23, a finger-control unit 24, afirst finger 25, a second finger 26, a control unit 27, amaximum-ratio-integration unit 28, a signal processing unit 29, and anaudio decoding unit 210.

A signal received at the antenna is supplied to the RF-conversion unit21, where the received signal is changed into an intermediate frequencysignal. The intermediate frequency signal is demodulated by theQPSK-demodulation unit 22, and, then, the demodulated signal is providedto the searcher 23, the first finger 25, and the second finger 26.

The searcher 23 includes a searcher-control unit 231, a correlation unit233, a peak-detection unit 234, and a timing-generation unit 235. Thesearcher-control unit 231 indicates a dispersion code to be searched forand a time span during which the search is to be conducted. Thecorrelation unit 233 detects a correlation between a pilot signal of acurrently used base station and a pilot signal of a surrounding basestation as these pilot signals are contained in the demodulated receivedsignals. The peak-detection unit 234 detects a peak in an output of thecorrelation unit 233, and the timing-generation unit 235 generates atiming signal indicative of a timing of the peak. The timing signal issupplied to the finger-control unit 24. The finger-control unit 24obtains a delay profile of the received signal of the currently usedbase station by using the timings reported from the searcher 23. In adescending order of the correlation in the delay profile, thefinger-control unit 24 notifies the first finger 25 and the secondfinger 26. Further, the finger-control unit 24 reports the receivedpilot strength of the surrounding base station to the control unit 27.

The first finger 25 and the second finger 26 have the sameconfiguration. Each finger includes a timing-synchronization unit 251, acorrelation unit 252, and a correlation-value detecting unit 253. Thecorrelation unit 252 calculates a correlation between the receivedsignal and the dispersion code that is specified by the control unit 27in advance. The correlation-value detecting unit 253 detects acorrelation value at a timing specified by the timing-synchronizationunit 251, and the detected correlation value is supplied to themaximum-ratio-integration unit 28. The maximum-ratio-integration unit 28attends to a maximum-ratio-integration process with respect to thecorrelation values supplied from the first and second fingers 25 and 26,and supplies the integrated signal to the signal processing unit 29. Thesignal processing unit 29 attends to error corrections, and the audiodecoding unit 210 reproduces audio from the error-corrected signals.Here, if the data output from the signal processing unit 29 is a controlmessage, the control message is supplied to the control unit 27.

In the related-art configuration as described above, a single stationmay have a plurality of cells using a plurality of radio frequenciesRF1, RF2, and RF3 as shown in FIG. 11. In such a case, a soft hand-offcan take place in any one of the radio frequencies RF1, RF2, and RF3,and, thus, hardware and software for providing a soft hand-off functionare required with respect to each radio frequency. Namely, every singleone of the selection units 13 ₀ through 13 _(m) of the base-stationcontroller needs to have a function to select one of the two receiveddata sets in order to achieve a soft-hand-off operation. This results inan undesirable cost increase.

Between adjacent base stations, areas covered by the same radiofrequency are overlapped at a peripheral portion. When a hard hand-offoperation is engaged because a soft hand-off is not available due tolack of resources, the mobile station 3 may move deep into a new cell toarrive at the point e while keeping communication with the base stationof an old cell. In such a case, signals transmitted from the basestation 2 appear to be nothing but noises to the mobile station 3.Further, the transmission signals of the base station 2 are strongerthan transmission signals coming from the base station 1 that iscurrently used. Namely, the signals transmitted from the base station 2interferes with communications of the mobile station 3 residing withinthe cell of the base station 1. These factors further limits the numberof mobile stations that can be used in the system.

As shown in FIG. 12, when the mobile station 3 having a connection withthe base station 1 is located at the point e, the mobile station 3 needsto transmit signals with such a strong power as to make them reach thebase station 1 by covering the distance r1. As far as the base station 2located only a distance r5 from the mobile station 3 is concerned, suchstrong transmission from the mobile station 3 at the point e is a sourceof interference against signals coming from other mobile stations. Thisfactor further limits the number of mobile stations that can be used inthe system.

In the related art, a soft hand-off operation should be usableregardless of what radio frequency is used by a mobile station. In thisconfiguration, a mobile station shifting a position thereof may comeclose to a neighboring base station, resulting in a change in a receivedpilot strength. Because of this, a delay profile needs to be constantlymonitored for all the radio frequencies with respect to the neighboringbase stations in addition to a delay profile of multi-path components.As a result, it is necessary to keep the searcher 23 in operation allthe time for monitoring purposes. This can be achieved, however, at acost of an increase in power consumption.

Accordingly, there is a need for a CDMA mobile communication systemwhich can accommodate a large number of mobile stations at a low costwhile providing soft hand-off services to the mobile stations.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea CDMA mobile communication system which can satisfy the need describedabove.

It is another and more specific object of the present invention toprovide a CDMA mobile communication system which can accommodate a largenumber of mobile stations at a low cost while providing soft hand-offservices to the mobile stations.

In order to achieve the above objects according to the presentinvention, a system for mobile communication based on code divisionmultiple access includes base stations, each of which communicates withmobile stations by using a plurality of radio frequencies coveringrespective cells, the respective cells including a first cell covered bya first radio frequency and a second cell covered by a second radiofrequency. The system further includes a base-station controller whichcommunicates with the base stations, and controls the mobile stations toswitch from the first cell of a first base station to the first cell ofa second base station via a soft hand-off operation and switch betweenthe first cell and the second cell within any base station via a hardhand-off operation, the base-station controller providing the mobilestations with no direct switch between the second cell of the first basestation and the second cell of the second base station.

In the system as described above, the plurality of radio frequencies areused for communication purposes, yet the number of radio frequenciespermitting a soft hand-off operation between adjacent base stations islimited. In this configuration, device elements on the base-station sidecan be simplified because there is no need for device elements toperform a soft hand-off function with respect to some of those radiofrequencies. This results in a lower device cost.

Further, a mobile station currently using a radio frequency that doesnot permit a soft hand-off operation can stop its search operation fromseeking pilot signals of surrounding base stations. This reduces powerconsumption in the mobile station.

According to another aspect of the present invention, the respectivecells covered by the plurality of radio frequencies have different areasizes (e.g., different radii). In this configuration, mobile stationscommunicating via one of the smaller cells can reduce transmission powerthereof compared to when communicating via one of the larger cells. Suchreduction in transmission power results in a decreased effect ofinterference on other mobile stations. Further, the mobile stationcommunicating via one of the smaller cells ends up keeping a distancefrom adjacent base stations. This mobile station thus suffers only alimited degree of interference from signals transmitted by surroundingbase stations. Consequently, the configuration of the present inventionincreases the number of mobile stations that can be accommodated by asingle base station.

In the manner as described above, the present invention can provide aCDMA mobile communication system which can accommodate a large number ofmobile stations at a low cost while providing soft hand-off services tothe mobile stations.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative drawing showing a CDMA mobile communicationsystem according to a principle of the present invention;

FIG. 2 is an illustrative drawing showing a CDMA mobile communicationsystem according to a first embodiment of the present invention;

FIG. 3 is a block diagram of a base-station controller according to thefirst embodiment of the present invention;

FIG. 4 is a block diagram of a buffer unit;

FIG. 5 is a block diagram of a receiver portion of a mobile station usedin a CDMA mobile communication system according to a second embodimentof the present invention;

FIG. 6 is an illustrative drawing showing a cell configuration of a CDMAmobile communication system according to a third embodiment of thepresent invention;

FIG. 7 is a block diagram of a base station according to the thirdembodiment of the present invention;

FIG. 8 is a block diagram of a RF-conversion unit used in the basestation of FIG. 7;

FIG. 9 is an illustrative drawing showing a cell configuration of a CDMAmobile communication system according to a fourth embodiment of thepresent invention;

FIG. 10 is an illustrative drawing showing a configuration of a typicalrelated-art mobile-communication system;

FIG. 11 is an illustrative drawing showing a configuration of cells ofrelated-art base stations that employ a CDMA scheme;

FIG. 12 is an illustrative drawing showing a hard hand-off operationperformed by a mobile station;

FIG. 13 is an illustrative drawing showing a soft hand-off operationperformed by a mobile station;

FIG. 14 is a block diagram showing a related-art configuration of a basestation and a base-station controller;

FIG. 15 is a block diagram of a RF-conversion unit of the base stationof FIG. 14;

FIG. 16 is a block diagram of a selection unit of the base-stationcontroller of FIG. 14; and

FIG. 17 is a block diagram of a receiver portion of a related-art mobilestation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, a principle and embodiments of the present inventionwill be described with reference to the accompanying drawings.

FIG. 1 is an illustrative drawing showing a CDMA mobile communicationsystem according to a principle of the present invention.

The system of FIG. 1 includes a CDMA base station 1, another CDMA basestation 2, and a CDMA mobile station 3. The base station 1 uses aplurality of radio frequencies RF1 and RF2 to cover respective areas,which are represented by ellipses in FIG. 1.

According to the present invention, the respective areas of the radiofrequencies RF1 and RF2 cover different ranges as shown in FIG. 1.Namely, the area of the radio frequency RF1 has a radius r1, and thearea of the radio frequency FR2 has a radius r2. Further, the area ofthe radio frequency RF1 of the base station 1 overlaps the area that thebase station 2 covers by using the same radio frequency RF1, therebymaking it possible to perform a soft hand-off operation between theseareas.

In the following, a description will be given with regard to a casewhere the mobile station 3 moves from the point a to the point e.

The mobile station 3 communicates with the base station 1 by using theradio frequency RF1 when it is located between the point a to the pointb. As the mobile station 3 arrives at the point b, a hard hand-offoperation is performed to switch from the radio frequency RF1 to theradio frequency RF2.

Between the point b and the point c, the mobile station 3 communicateswith the base station 1 by using the radio frequency RF2. When themobile station 3 comes to the point c, a hard hand-off operation iscarried out to switch from the radio frequency RF2 to the radiofrequency RF1.

The mobile station 3 uses the radio frequency RF1 between the point cand the point d. As the mobile station 3 moves away from the basestation 1 and reaches a certain point between the point c and the pointd, the mobile station 3 engages in a soft hand-off operation so as tocommunicate with both the base station 1 and the base station 2. Whenthe mobile station 3 reaches a certain point sufficiently far away fromthe base station 1, the mobile station 3 disengages from the softhand-off operation, and communicates only with the base station 2 byusing the radio frequency RF1.

As it comes to the point d, the mobile station 3 performs a hardhand-off operation to switch from the radio frequency RF1 to the radiofrequency RF2. Thereafter, the mobile station 3 uses the radio frequencyRF2 to communicate with the base station 2.

In this manner, a mobile communication system of the present inventionhas a plurality of cells using respective radio frequencies and havingrespective area sizes (e.g., respective radii). The respective areasizes may be different from each other, and the radio frequency coveringthe largest area may have a cell that overlaps a counterpart cell of anadjacent base station. A soft hand-off operation between base stationsmay be performed only with respect to this radio frequency that coversthe largest area. Within the same base station, a hard hand-offoperation is performed to switch between different radio frequencies.Between different-base stations, a soft hand-off operation is carriedout to move from one station to another.

According to this configuration, a soft hand-off operation may beperformed only with respect to the radio frequency of the largest cell,so that communication lines for other radio frequencies do not need asoft-hand-off function. Devices used on the base-station side can bethus simplified in terms of their circuit configurations.

A trigger may be necessary to initiate a hard hand-off operation at anyone of the points b, c, and d as described above. To this end, the basestation may periodically require the mobile station 3 to report receivedpilot strengths (e.g., may send a pilot measurement request to themobile station 3). When receiving a pilot strength measurement messagefrom the mobile station 3, the base station may estimate a position ofthe mobile station 3 based on the received message. If the position isfound to be close to a cell boundary, an instruction to perform a hardhand-off operation is sent to the mobile station 3 to trigger a switch.

Alternatively, the mobile station may be instructed to constantlymonitor received pilot strengths of surrounding base stations. Themobile stations may report a change of a received pilot strength as itcrosses a predetermined threshold.

As previously described, the related-art CDMA-mobile-communicationsystem allows mobile stations using any radio frequencies to perform asoft hand-off operation, so that all the mobile stations using any radiofrequencies can move from one base station to another base stationwithout disrupting their continuous communications. Such a systemconfiguration tends to be costly. Further, since a hard hand-offoperation is performed when requirements for a soft hand-off operationare not satisfied, such a hard hand-off operation interferes withcommunications of other mobile stations, limiting the number of mobilestations that can be accommodated within a single base station.

According to the principle of the present invention, a soft hand-offoperation between adjacent base stations is performed only by using aselected radio frequency (RF1), and a hard hand-off operation isperformed to switch between different radio frequencies (RF1 and RF2)within the same base station. In this configuration, resources (e.g.,circuits such as those of selection units) of the base-stationcontroller allocated to a mobile station currently using the radiofrequency RF2 do not need such hardware and software as required forsoft hand-off operations. This simplifies device configurations.

Where the area covered by the radio frequency RF2 has a radius smallerthan that of the area covered by the radio frequency RF1, signalstransmitted from the base station using the radio frequency RF2 can havesmaller signal strength. This makes it possible to reduce powerconsumption in the high-power amplifier of the base station, and, also,reduces interference between adjacent cells. This helps to increase thenumber of mobile stations that can be accommodated in the base station.

With the related-art configuration, a mobile station needs to constantlymonitor pilot strengths of surrounding base stations so as to be readyfor a soft hand-off operation no matter what radio frequency iscurrently used. In the present invention, on the other hand, a mobilestation using the radio frequency RF2, for which no soft hand-offoperation is performed, can stop the monitoring operation of searchingfor pilot signals of surrounding base stations. That is, all that thesearcher needs to do is to search for a delay profile of the multi-pathcomponents. This reduces power consumption in the mobile station.

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings.

First Embodiment

FIG. 2 is an illustrative drawing showing a CDMA mobile communicationsystem according to a first embodiment of the present invention.

The same reference numerals as those of FIG. 12 are used in FIG. 2. FIG.2 shows a cell configuration at the top, pilot strengths received by themobile station 3 in the middle, and a base station and a radio frequencythat the mobile station 3 is currently using at the bottom.

Each of the base stations 1 and 2 uses the radio frequency RF1 and RF2to cover respective areas (cells). The area of the radio frequency RF1has a radius r1 with a base station at a center thereof, and the area ofthe radio frequency RF2 has a radius r2 with the base station at acenter thereof. The areas covered by the radio frequency RF1 areoverlapped at a peripheral portion thereof between the base stations 1and 2, thereby permitting a soft hand-off operation in the overlappingarea.

In what follows, a description will be given with regard to a case inwhich the mobile station 3 moves from the point a to the point h in FIG.2.

At the point a, the mobile station 3 starts communicating with the basestation 1. The radio frequency RF1 is initially used. As the mobilestation 3 moves toward the point b, a received pilot strength of theradio frequency RF1 gradually increases. At the point b, the receivedpilot strength of the radio frequency RF1 of the base station 1 exceedsz, which indicates a received pilot strength of the radio frequency RF1that is observed when the mobile station 3 enters the cell of the radiofrequency RF2. As this happens, the mobile station 3 reports this to thebase-station controller 4 via the base station 1.

Upon receiving the report, the base-station controller 4 instructs themobile station 3 to perform a hard hand-off operation to switch to theradio frequency RF2. In response, the mobile station 3 carries out ahard hand-off operation at the point b, and, thereafter, uses the radiofrequency RF2 to communicate with the base station 1.

As the mobile station 3 moves further and comes closer to the point c, areceived pilot strength of the radio frequency RF2 of the base station 1decreases. Eventually, the received pilot strength of the radiofrequency RF2 becomes smaller than x, which indicates a pilot strengthusable for communication. The mobile station 3 notifies the base-stationcontroller 4 via the base station 1.

The base-station controller 4 instructs the mobile station 3 to performa hard hand-off operation to switch to the radio frequency RF1. Inresponse, the mobile station 3 carries out a hard hand-off operation atthe point c, and, thereafter, uses the radio frequency RF1 tocommunicate with the base station 1.

As the mobile station 3 further moves toward the point d, the radiofrequency RF1 of the base station 2 appears with an increasing pilotstrength. The received pilot strength of the radio frequency RF1 of thebase station 2 eventually exceeds y, which indicates a pilot strength totrigger a hand-off operation. As this happens, the mobile station 3notifies the base-station controller 4 via the base station 1.

The base-station controller 4 sends a resource request to the basestation 2 with an aim of performing a soft hand-off operation, andinstructs the base station 2 and the mobile station 3 to perform a softhand-off operation as resources are secured. As a result, the mobilestation 3 communicate with both the base station 1 and the base station2. The base-station controller 4 selects one of two received data setswhich has the best quality as one data set is received from the basestation 1 and the other data set is received from the base station 2.The selected data set is sent to the mobile-switch center 5.

As the mobile station 3 moves further and comes close to the point f,the received pilot strength from the base station 1 becomes weak.Eventually, the received pilot strength of the radio frequency RF1 ofthe base station 1 falls below x, which is a pilot strength usable forcommunication. The mobile station 3 reports this to the base-stationcontroller 4 via the base stations 1 and 2.

The base-station controller 4 instructs the base stations 1 and 2 andthe mobile station 3 to finish the soft hand-off operation. As a result,the mobile station 3 communicates only with the base station 2 by usingthe radio frequency RF1.

As the mobile station 3 moves toward the point g, a received pilotstrength of the radio frequency RF1 of the base station 2 graduallyincreases. In the end, the received pilot strength exceeds z, which isdefined as the received pilot strength of the radio frequency RF1 thatis observed when the mobile station 3 enters the cell of the radiofrequency RF2. As this happens, the mobile station 3 reports this to thebase-station controller 4 via the base station 2.

In response, the base-station controller 4 instructs the mobile station3 to perform a hard hand-off operation so as to switch to the radiofrequency RF2. The mobile station 3 carries out the hard hand-offoperation at the point g.

The hand-off operations as described above are repeated as the mobilestation 3 shifts its position from cell to cell.

FIG. 3 is a block diagram of a base-station controller according to thefirst embodiment of the present invention.

The base-station controller shown in FIG. 3 differs from that of therelated-art in configurations of the selection units 13. In the relatedart, every one of the selection units 13 ₀ through 13 _(m) of thebase-station controller 4 has the configuration shown in FIG. 16. In thepresent invention, selection units 13 ₀ through 13 _(n) have the sameconfiguration as that of FIG. 16, and are used for mobile stations 3currently using the radio frequency RF1. In addition, buffer units 14 ₀through 14 _(m) are allocated to the mobile stations 3 currently usingthe radio frequency RF2.

FIG. 4 is a block diagram of a buffer unit 14. The buffer unit 14 is anyone of the buffer units 14 ₀ through 14 _(m).

As shown in FIG. 4, the buffer unit 14 includes only the first buffer131, the third buffer 133, the audio decoding unit 134, the audio codingunit 135, and the buffer-control unit 136. Other elements present in theconfiguration of FIG. 16 such as the demultiplexer 137, the first checkunit 138, the second check unit 139, the second buffer 132, and theselector 140 are removed. Namely, the buffer unit 14 includes the firstbuffer 131 and the audio decoding unit 134 on the receiver side thereof,and includes the third buffer 133 and the audio coding unit 135 on thetransmitter side thereof. The buffer unit 14 has such a simplifiedconfiguration because the mobile stations 3 currently using the radiofrequency RF2 do not perform soft hand-off operations and there is noneed for the buffer unit 14 to be equipped with a function to perform asoft hand-off operation.

The base-station controller 4 needs to switch between use of theselection unit 13 and use of the buffer unit 14′ as a hard hand-offoperation is performed between the radio frequency RF1 and the radiofrequency RF2, and such a switch needs to be made at the same timing asthe hard hand-off operation. This switching function may be provided byutilizing a communication-line switching function of the communicationsetting unit 12 and the MSC-connection unit 15, which are present in therelated-art system.

As described above, the selection function of the selection unit 13needs to be provided only for a portion relevant to the radio frequencyRF1. This is because only the mobile stations 3 using the radiofrequency RF1 can perform a soft hand-off operation. Since other mobilestations 3 using other radio frequencies do not need a soft hand-offfunction, such a selection function is not necessary for portionscorresponding to the other frequencies (e.g., RF2). In the portionscorresponding to the other frequencies, therefore, the number of deviceelements can be reduced by removing the demultiplexer, the check units,one of the buffers, and the selector.

The present invention is not limited to the configuration as describedabove in which the system uses only two radio frequencies. It isapparent that the present invention is equally applicable to aconfiguration where the system uses more than two radio frequencies.

Second Embodiment

FIG. 5 is a block diagram of a receiver portion of a mobile station usedin a CDMA mobile communication system according to a second embodimentof the present invention. In FIG. 5, the same elements as those of FIG.17 are referred to by the same numerals, and a description thereof willbe omitted.

A configuration of FIG. 5 differs from that of FIG. 17 only in asearcher-stop-control unit 232 is newly provided in the searcher 23.

The searcher-stop-control unit 232 blocks a function of the searcher 23(i.e., stops the operation of the searcher 23) in response to aninstruction from the control unit 27 when the function of the searcher23 is to search for pilot signals of surrounding base stations. Theblocking of the function is effected when the mobile station 3 uses theradio frequency RF2, and, thus, does not perform a sort hand-offoperation.

The searcher 23 is generally responsible for two functions. One is tosearch for pilot signals of surrounding base stations, and the other isto search for multi-path components of communicated signals. In thesecond embodiment of the present invention, a mobile station using aradio frequency that permits no soft hand-off operation does not searchfor pilot signals of the surrounding base stations, and only searchesfor multi-path components. This reduces the load on the mobile station 3in terms of use of hardware and software thereof, thereby achieving areduction in power consumption.

Radio frequencies that do not permit a soft hand-off operation may bereported to the mobile station 3 as configuration information inadvance, or may be reported to the mobile station 3 by a message sentfrom one of the base stations 1 and 2 and the base-station controller 4.In the latter case, the mobile station 3 does not have to haveidentifications of radio frequencies that do not permit a soft hand-offoperation, but can acquire the identifications through messages sentfrom the base stations. This provides flexibility for changes in thesystem configuration.

Third Embodiment

FIG. 6 is an illustrative drawing showing a cell configuration of a CDMAmobile communication system according to a third embodiment of thepresent invention. FIG. 7 is a block diagram of a base station accordingto the third embodiment of the present invention. FIG. 8 is a blockdiagram of a RF-conversion unit used in the base station of FIG. 7.

As shown in FIG. 6, the base stations 1 and 2 use a radio frequency RF3for wireless communication in addition to the radio frequencies RF1 andRF2. An area covered by the radio frequency RF3 has the radius r2 thesame as that of the area covered by the radio frequency RF2.

As shown in FIG. 7, a base station of the third embodiment includes theRF-conversion units 301 through 30 ₃ used for the radio frequencies RF1through RF3, respectively. In the third embodiment, the RF-conversionunits 30 ₂ and 30 ₃ have a configuration as shown in FIG. 8, and differsfrom that of the RF-conversion unit 30 ₁ in that a switch 307 is newlyprovided. The switch 307 serves to switch on/off an input to thehigh-power amplifier 308 in response to an instruction from theBTS-control unit 33. This makes it possible to switch on/offcommunications by the radio frequencies RF2 and RF3.

In the base station, the BTS-control unit 33 knows the number of mobilestations 3 currently engaging in a call with respect to each radiofrequency. This information is provided as communication-line-settinginformation. In the configuration of FIG. 7, the BTS-control unit 33 isprovided with a function to detect the number of mobile stations 3 usingthe radio frequency RF1 and currently engaging in a call. Depending onthe detection result, transmission of the radio frequency RF2 from theRF-conversion unit 302 is either switched on or switched off.

Initially, communications with the mobile stations 3 are conducted byusing only the radio frequency RF1. When the number of mobile stations 3using the radio frequency RF1 and currently engaging in a call increasesand approaches to an upper limit thereof, transmission using the radiofrequency RF2 is commenced. At the same time, the base station has partor all of the mobile stations 3 report received pilot strengths of theradio frequency RF1 as long as the mobile stations 3 are currentlyengaging in a call and using the radio frequency RF1. If a pilotstrength received by a given mobile station 3 is greater than a giventhreshold, it is ascertained that this mobile station 3 is positionedsufficiently close to the base station, i.e., is positioned within thecell of the radio frequency RF2. In this case, an instruction is sent tothis mobile station 3 to perform a hard hand-off operation to switch tothe radio frequency RF2. In this manner, the number of mobile stations 3receiving services via the radio frequency RF1 is reduced, therebymaking room for additional mobile stations 3.

By the same token, the BTS-control unit 33 of the base station isprovided with a function to detect the number of mobile stations 3receiving services via the radio frequency RF2. Depending on thedetected number, transmission of the radio frequency RF3 is switched onor off. This insures that the radio frequency RF2 has room to accept newmobile stations 3 switching from the radio frequency RF1 via hardhand-off operations.

Some measures may be taken in order to prevent transmission of the radiofrequencies RF2 and RF3 from switching on/off too frequently. Forexample, the number of mobile stations 3 for triggering or stoppingtransmission of the radio frequencies RF2 and RF3 may be given ahysteresis characteristic, or may be disregarded for a predeterminedtime period.

Under such control as described above, transmission of the radiofrequencies RF2 and RF2 are stopped to render communication services byusing only the radio frequency RF1 when only a small number of mobilestations 3 are engaging in a call via a base station. This reduces powerconsumption in the base station. Further, this configuration can reducean interfering effect on other mobile stations using other basestations.

In the third embodiment described above, the two radio frequencies RF2and RF3 are used via hard hand-off switching. The present invention isnot limited to this configuration, but is applicable to a case whereonly one radio frequency (e.g., RF2) is used via a hard hand-offoperation. When the number of mobile stations 3 is small, only the radiofrequency RF1 is transmitted. As the number of the mobile stations 3increases, the radio frequency RF2 is transmitted to allow the mobilestations 3 to switch from the radio frequency RF1 to the radio frequencyRF2 via a hard hand-off operation. Further, the present invention isequally applicable to a case where more than two radio frequencies areused via hard hand-off operations.

Fourth Embodiment

FIG. 9 is an illustrative drawing showing a cell configuration of a CDMAmobile communication system according to a fourth embodiment of thepresent invention.

In the fourth embodiments, two radio frequencies RF2 and RF3 are used asin the third embodiment, but they have different area sizes from eachother in contrast to the same area size of the third embodiment. An areacovered by the radio frequency RF2 has a radius r2, and an area coveredby the radio frequency RF3 has a radius r3 smaller than the radius r2.In the fourth embodiment, further, transmission of the radio frequenciesRF2 and RF3 is not controlled in terms of switching on/off thereof.

With the smaller radius r3 of the radio frequency RF3 compared with thatof the radio frequency RF2, the mobile stations 3 are switched from theradio frequency RF2 to the radio frequency RF3 if the mobile stations 3currently using the radio frequency RF2 are positioned sufficientlyclose to the base station. Because of the smaller radius r3 of the radiofrequency RF3, transmission power of the base station can be smaller forthe radio frequency RF3, thereby achieving a reduction in powerconsumption.

Various modifications can be made to the embodiments of presentinvention. The embodiments have been described with reference toexamples in which two or three radio frequencies are used. The presentinvention is not limited to these examples, but is applicable to use ofany larger number of radio frequencies.

The number of radio frequencies (e.g., RF1) permitting a soft hand-offoperation is not limited to one, but can be more than one. What isimportant is to provide radio frequencies (e.g., RF2 and RF3) offeringno soft hand-off functions in addition to radio frequencies (e.g., RF1)permitting a soft hand-off operation. With this configuration, softhand-off operations are performed only with respect to the radiofrequencies (e.g., RF1) that permit a soft hand-off operation.

Further, the embodiments have been described with reference to a casewhere the areas covered by the hard-hand-off radio frequencies RF2 andRF3 are smaller than the area covered by the soft-hand-off radiofrequency RF1. The present invention is not limited to thisconfiguration, but is applicable to a case where all the areas have thesame area size. In such a case, conditions that trigger hard hand-offswitching from the radio frequency RF1 to the radio frequency RF2 may bedetermined as they are appropriate. This configuration is based on apremise that the number of radio frequencies permitting soft hand-offoperations should be limited. With such a configuration, selection unitsof a base-station controller and searchers of mobile stations can besimplified in terms of structures thereof although no effect is expectedto bring about an increase in the number of mobile stations that can beaccommodated in a base station.

The present invention is applicable to any system that performs anyhand-off operations similar to those described above, and a type ofhand-off operation is not limited to that of IS-95*A.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

The present application is based on Japanese priority application No.10-297709 filed on Oct. 20, 1998, with the Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

1.-12. (canceled)
 13. A base station which forms a first area and a second area by utilizing a first frequency and a second frequency, respectively, used in common with an adjacent base station, comprising a radio unit configured to set the first area such that the first area overlaps an area that is formed by the adjacent base station by use of the first frequency, and to set the second area smaller than the first area such that the second area is formed within the first area, wherein soft handover from the adjacent base station using the first frequency used in common is permitted, and handover from the adjacent base station using the second frequency used in common is not permitted. 